Abstract

Direct injection of natural gas (DING) is an attractive alternative fuel concept for internal combustion engines because of its potential for low pollutant emissions, high efficiency, and high power density. However, injector designs need be optimized to maximize performance and minimize emissions, which will require a better understanding of the fuel/air mixing processes. Therefore, a research program was initiated to use laser diagnostic techniques to measure the fuel concentration within the cylinder of a DING engine. The engine was modified for optical access by installing an enlarged spacer plate with three fused silica windows, elongating the connecting rod to maintain the compression ratio, and replacing one of the exhaust valves with an imaging window.

The injection and mixing processes were examined using planar laser-induced fluorescence (PLIF) of acetone-seeded natural gas to obtain qualitative maps of the fuel/air ratio. Initial acetone PLIF images were acquired at 0.1 ms intervals in a quiescent combustion chamber with the piston locked in a position corresponding to 90$\sp\circ$ BTDC. Additional images were acquired with 0.5 crank angle degree resolution in a motoring engine at 600 rpm for injections starting at 30, 20, and 15$\sp\circ$ BTDC. Single-laser-shot measurements of the fuel/air ratio in the cylinder of a motored DING engine were obtained using a dual-pump coherent anti-Stokes Raman scattering (CARS) technique capable of simultaneously probing N$\sb2$ and CH$\sb4.$ CARS measurements were acquired at eight probe volume locations with 1.0 crank angle degree resolution for injections starting at 30 and 20$\sp\circ$ BTDC.

A set of scaling factors derived from the conservation equations effectively collapsed fuel jet penetration measurements under both motoring and stationary piston conditions. The scaling factors are based on the pseudo diameter of the jet, which is the diameter of an equivalent sonic jet expanded to match the in-cylinder conditions. The penetration of the fuel jets compared favorably to a two-region vortex ball model of jet penetration. The CARS measurements show large fluctuations in fuel concentration on a shot-to-shot basis, which are consistent with the large-scale mixing structures evident in the PLIF images.